The invention relates to a process and to an arrangement for producing tetrahydrofuran from a hydrous butane diol solution contaminated by light volatile organic compounds.
Processes for producing tetrahydrofuran from hydrous butane diol solutions have been known for some time.
The mentioned processes for producing tetrahydrofuran from a hydrous butane diol solution have in common that the hydrous raw butane diol solution, which as a rule contains approximately 20 to 50% butane diol in water, is first purified in a multistage distillation from undesirable low-boiling and high-boiling constituents, including water. Subsequently, the now anhydrous pure butane diol is catalytically converted on a packed-bed catalyst to tetrahydrofuran and water. The tetrahydrofuran product solution containing water and other low-boiling and high-boiling constituents is subsequently processed again in several stages in a distillative manner to pure tetrahydrofuran.
From German Patent Document DE-OS 25 09 968, a process is known for producing tetrahydrofuran in which the hydrous raw butane diol solution is dehydrated in the liquid phase. As dehydration catalysts, inorganic acids, such as H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4 are used. However, these inorganic acids are difficult to handle because they cause, for example, increased corrosion, and their disposal presents problems.
Another process for producing tetrahydrofuran is the so-called Reppe process. In the Reppe process, the conversion of hydrous butane diol solution takes place by way of an acidic catalyst, as a rule H.sub.3 PO.sub.4, in the liquid phase at temperatures of above 250.degree. C. and pressures of approximately 100 bar. To implement this process requires the lay-out of a reactor or reactors to obtain high pressure, as well as corresponding condenser units. In addition, the process results in significant quantities of undesirable by-products which, on the one hand, have a gluing effect on the acidic catalyst and, on the other hand, cannot be separated or can only be separated poorly. A detailed description of the Reppe process can be found, for example, in BIOS-Report No. 367 (1945), FIG. 14.
The above-mentioned processes for producing tetrahydrofuran from a hydrous butane diol solution have in common that the hydrous raw butane diol solution, which as a rule contains approximately 20 to 50% butane diol in water, is first purified in a multi-stage distillation--partly in vacuum columns requiring high investment and operating costs--from undesirable low-boiling and high-boiling constituents, including water. Subsequently, the now anhydrous pure butane diol is catalytically converted on a packed-bed catalyst to tetrahydrofuran and water. The tetrahydrofuran product solution containing water and other low-boiling and high-boiling constituents is subsequently processed again in several stages in a distillative manner to pure tetrahydrofuran. Since, in the known process corresponding compounds are formed again, during the hydration of butane diol comparable purifying or separating steps are carried out twice.
As far as the process is concerned, these required purification and separation steps require comparatively high expenditures because they each consist of several distillation columns arranged behind one another. In addition to high operating and investment costs, this process has an additional disadvantage that, because of thermal stress during the individual distillation steps and a loss of catalyst selectivity in the anhydrous state, there is an increased occurrence of by-products, which results in tetrahydrofuran product losses.
Those processes for producing tetrahydrofuran which avoid such high-expenditure purification and separation steps, as a rule, work with free acids, which is unfavorable economically as well as with respect to protecting the environment. For example, requirements include: a high-expenditure reactor concept, corrosion problems must be taken into account and acid removal or neutralization requires at least one additional purification step. Acid waste disposal is also a problem.
It is an object of the present invention to provide a process and arrangement for producing tetrahydrofuran, in which the above-mentioned disadvantages can be avoided and the overall expenditures for such a system can therefore be reduced.
According to the invention, this is achieved in that
a) light volatile organic compounds contained in the hydrous butane diol solution are removed by distillation;
b) then the thus prepurified hydrous butane diol solution is dehydrated over an acidic aluminum oxide catalyst; and
c) the thus obtained high-tetrahydrofuran fraction is distilled in one or more stages for obtaining pure tetrahydrofuran.
As mentioned initially, the invention also relates to an arrangement for producing tetrahydrofuran from a hydrous butane diol solution contaminated by light volatile organic compounds.
For achieving the above-mentioned object of the invention, an arrangement is suggested which has
a) at least one distillation column for removing light volatile organic compounds from a hydrous butane diol solution;
b) a reactor (R) connected behind the distillation column(s), particularly a packed-bed reactor, in which dehydration of hydrous butane diol solution to a high-tetrahydrofuran fraction takes place; and
c) an after purification system, which distills in one or more stages, and is used for obtaining pure tetrahydrofuran from the high-tetrahydrofuran fraction drawn from the reactor.